An example drawn from horology will make readily understandable to what end the object of the present invention may be employed, such object being readily usable in many other known domains as for instance that of photography (zoom lenses) or equipment in general.
In its first embodiment the invention described in patent document CH-A-432 388 has as object a watch case comprising two elements which may be oriented in any desired manner relative to one another, one comprising a first annular element bearing a glass and the other a second annular element on which the first is rotatively mounted, a packing being inserted between these two elements.
FIG. 1 of the present invention shows such prior art, somewhat modified relative to the cited document, but however remaining equivalent as far as concerns the form and arrangement of the packing.
The cross-section of FIG. 1 shows that the watch case includes a caseband 8, a back cover 13, a movement 7 housed in the case and a dial 6 surmounting movement 7. An assembly 3 including a bezel 1 exhibiting an annular interior shoulder 20, a crystal 2 fitted to the bezel by means of a first packing 4 and an annular element forming a flange 5, may be driven in rotation relative to the caseband and to the dial.
Between the bezel and the caseband is squeezed a second round packing 9 which assures sealing of the case. Packing 9 is arranged between two coaxial cylindrical surfaces, one 22 of which bounds a portion of the bezel and the other 23 a portion of the caseband. A casing ring 10-11 arranged within caseband 8 presents an annular collar 21 resting on the shoulder 20 of the bezel I so as to engage or retain the rotatable assembly 3 on caseband 8. The caseband 8 shows in its lower portion a collar 24 extending towards the center of the case. On this collar 24 bears the casing ring 10-11. Screws 31 vertically positioned and traversing bores 25 provided in collar 24 are screwed into threads 26 provided in the ring in order to fix the ring to the caseband.
As mentioned hereinabove, packing 9 exhibits a round cross-section. From its manufacturing process, it also exhibits two diametrically opposed injection burrs symbolized on the figure by two dashes 14 and 15. In the dimensions used here, experience has shown when the bezel 1 is rotated, packing 9 rolls or twists on itself in an irregular manner so that the burrs 14 and 15 form helices over the entire periphery of the packing, thus creating a track between the outer world and the interior of the case, this route being used by water in order to penetrate the mechanism within the case.
It has been determined that the phenomenon of twisting may disappear if there be adopted for the O-ring a relationship between the interior diameter Di of the ring and the diameter Dt of the cross-section of the thread constituting the ring, which is less than a limit value which may be estimated at 20. Thus, for an O-ring currently employed in micromechanics and which shows a diameter Di comprised between 0.8 and 5 mm and a diameter Dt comprised between 0.4 and 0.5 mm, such relationship is at worst 5/0.4=12.5 and this O-ring will not twist. On the other hand, in the example shown on FIG. 1, O-ring 9 has an interior diameter Di of 38 mm and a cross-section Dt of 0.7 mm hence a relationship Di/Dt=38/0.7=57.3 highly unfavourable.
In this latter case, since the diameter Di of 38 mm is imposed by the design, it would be necessary to provide a thread cross-section of at least 1.9 mm to arrive at a relationship of 20 and avoid twisting of the packing. However, such arrangement will have as consequence an increase in the overall diameter of the timepiece.
The toroidal packing 9 shown on FIG. 1 additionally shows a second drawback. Effectively, through the slot referenced 19 on the figure, there may be introduced not only water, but also dust which by its abrasive action may damage the packing. The packing then runs the risk of no longer sealing out water. To provide a solution to this problem, certain designers provide two toroidal packings, one on top of the other, the first retaining dust and the second--which remains intact--retaining water. However, here the solution is both difficult and cumbersome.
The packing ring described in patent document FR-A-1 185 726 exhibits a polygonal cross-section having two regions separated by a depression which may recall a dumbbell configuration. However, the axis of such dumbbell is shown perpendicular to the axis of the ring in a manner such that if this latter is employed to seal a rotating bezel, one would end up with a prohibitive increase in the overall diameter of the timepiece . On the other hand, mounted between the caseband 8 and bezel 1 of FIG. 1, such ring would urge annular collar 21 onto shoulder 20 with a force such that the bezel could no longer be driven in rotation except at the price of a very substantial force not to mention the risk of seizing up.